Skeletal muscle undergoes a progession of morphological and cytological changes during old age which culminates in loss of muscle bulk and function. Concurrent with these alterations is an apparent reduction in regeneration potential. This study investigates the nature of the morphological and cytological alterations in senile muscle through light and electron microscopic quantitation with an aim toward establishing the environmental influences on the developmental fate of satellite cells in senile muscle. The intrinsic behavior of young/old satellite cells and influence of environment will be studied in young/old muscle through auto and reciprocal transplants of entire muscles or implants of isolated satellite cells. In addition, the hypothesis that many senile-related alterations in skeletal muslce are a result of decreased levels or activity of neurotrophic factors will be tested directly in vitro and in vivo.